Transformer Inrush Currents Research Articles

The Impact of Zero-Mode Inrush Current of T-Hin on Zero-Sequence Overcurrent Protection and an Improved Protection with the Second Harmonic Restraint

In recent years, the zero-mode inrush current of high-impedance transformer with built-in high-voltage winding (T-Hin), which has large amplitude and decays slowly, causes the misoperation of zero-sequence overcurrent protection. Compared with magnetizing inrush current, the waveform of zero-mode inrush current is inconsistent and irregular, and few researches have proposed the mathematical analysis as well as the improved protection using waveform characteristics. In this paper, the mathematical expression of transformer zero-mode inrush current is derived. Further considering the parameter differences, the zero-mode inrush current of T-Hin is larger, which tends to cause the misoperation. The mathematical waveforms fit well with the recorded waveforms. Both recorded waveforms and mathematical waveforms in various conditions prove that the second harmonic ratio (the ratio between the second harmonic and first harmonic) of zero-mode inrush current is significant. Based on the above analysis, a criterion based on the second harmonic ratio restraint of zero-mode inrush current is proposed. If the second harmonic ratio exceeds the setting value, it is considered that the inrush current is generated and sends a signal to restrain the protection. The theoretical setting value of the proposed criterion and the practical engineering method for determining the setting value are obtained.

Systematization of the Simulation Process of Transformer Inrush Current Using EMTP

An inrush current is generated when a transformer is energized. This current has a large magnitude and rich harmonics, thereby causing mal-operation of the protection relay. Therefore, the development of countermeasures against inrush current is necessary, and this study has been performed by computer simulations. However, it is difficult for a power system operator to perform a computer simulation as it is difficult to determine what data should be selected and entered. Therefore, this paper establishes the simulation process of transformer inrush current using the Electromagnetic Transients Program (EMTP). Two methods to simulate the transformer inrush current are described in detail. Based on the actual 154 kV transformer test report in Korea, the simulation results of the inrush current using the two methods are discussed.

Mitigation Transformer Inrush Current Using Modified Transient Current Limiter

This study presents a modified transient current limiter (MTCL) for mitigation the inrush current of transformers. The MTCL is based on conventional transient current limiter (TCL), which, its configuration is modified to overcome the TCL drawbacks of operation. The proposed MTCL offers lower power losses and voltage/current THD during normal operation mode. It needs only one limiting reactor instead of two limiting reactors, which results in cost saving. The theoretical analysis of the MTCL for suppressing the inrush current has been presented and the performance was tested by PSCAD/EMTDC simulation anda experimental prototype. Both simulation and experimental results showed that the MTCL is effective for suppressing the transformer inrush current. Also, the capability of the MTCL for suppressing the transformer inrush current was compared with the conventional TCL.

Compliance of a Generic Type 3 WT Model with the Spanish Grid Code

The expansion of wind power around the world poses a new challenge that network operators must overcome, namely the integration of this renewable energy source into the grid. Comprehensive analyses involving time-domain simulations must be carried out to plan network operation and ensure power supply. In light of the above, and with the aim of extending the use of the wind turbine models developed by Standard IEC 61400-27-1 and assessing their performance according to national grid code requirements, an IEC Type 3 wind turbine model has been submitted for the first time to Spanish grid code PO 12.3. Indeed, there is a lack of studies submitting generic wind turbine models to national grid code requirements. The model’s behavior is compared with field measurements of an actual Gamesa G52 machine and with its detailed simulation model. The outcomes obtained have been comprehensively analyzed and the results of the validation criteria highlight that several modeling modifications, in the cases of non-compliance, must be implemented in the IEC-developed Type 3 model in order to comply with PO 12.3. Nevertheless, the results also show that when the transformer inrush current is not considered, the reactive power response of the generic Type 3 WT model meets the validation criteria, thus complying with Spanish PO 12.3.

The influence of power supply network inductance on the HTS transformer inrush current

The influence of power supply network inductance on the HTS transformer inrush current

Investigations into the transformer inrush current problem

A transformer being energised may draw a large transient current from the grid supply, resulting in a temporary voltage dip at the point of connection (POC) where customers are connected. The voltage dip is dependent upon the magnitude of the transformer inrush current. The peak current of the first cycle, under worst conditions, is considered important. This paper presents the results achieved following the energisation of a 10MVA 132/11kV transformer as well as the practical mitigation measures to minimise the impact of the transformer energisation.Keywords: transformer saturation; transformer inrush current modelling; voltage drop; PSCAD software

Comparison and Analysis of Magnetizing Inrush and Fault Condition for Power Transformer

This paper presents the second harmonics present in the primary current of a power transformer at different conditions using Fast Fourier Transform and Total Harmonic Distortion techniques to analyze the inrush condition and to distinguish it with fault condition of a power transformer. Result shows that the 2nd harmonic content is pre-dominant in inrush condition of primary current of the power transformer. It is observed that there are significant differences amongst the parameters found during inrush condition, normal condition and internal fault condition which are useful in the identification of magnetizing inrush current of power transformer. The simulation is done in MATLAB/SIMULINK.

Optimization of Losses in a Three-Phase Transformer Design with Inrush Current Analysis

The proposal of this work is the study of the design of a three-phase distribution transformer and its optimization using techniques from natural differential evolution along with particle swarm optimization algorithms in order to minimize total losses and look at the inrush current. Transformer parameters were calculated analytically using the OCTAVE software, and the results obtained were used to define the geometry of the project and to carry out the following simulations: the first through the finite element method Finite Element Method Magnetics software, which analyzed the magnetic flux densities of the transformer core, and the second through Alternative Transient Program software for analyzing transformer inrush current. The analysis of the highest peak value of the energizing current of the three-phase transformer for the reference design and of the new optimized project is carried out by means of an approximate estimative analytical method applied to single-phase transformers and adapted in this research for use in three-phase transformers via analytical calculation and transient simulation. The verifications made through field results and measurements show that the methodology used for three-phase transformers is very efficient, minimizing losses with a consequent decrease in the energizing current.

Eliminasi Arus Inrush Transformator Pada Beban Motor Induksi 3 Fasa Menggunakan Metode Pre–Insertion Resistor

Transformer is the most widely used electromagnetic induction equipment in the electrical system. When the transformer is connected to a voltage source it will happen an undesirable phenomenon that is inrush current. The incidence of inrush current in the transformer can reduce the life of the transformer because large currents will enhance the performance of the transformer. One method to reduce inrush current is to use pre-insertion resistor method. This research uses pre-insertion resistor method which is a method with insert resistor on the primary side of transformer with time 0.18 second. The resistor used in the pre-insertion resistor method serves as a current buffer when the process of energizing the transformer. This research uses Matlab Rb2014a software to know the inrush current of transformer. The result of the transformer simulation test using pre-insertion resistor method, the inrush current is reduced by 29% for the 710 kW motor load capacity, with the motor load capacity of 1420 kW reduced inrush current of 30%. Furthermore, with a load capacity of 2130 kW motor, the inrush current is reduced by 28% and for the motor load capacity 3550 kW reduces the current by 27%.

Analysis of inrush currents of the unloaded transformer using the circuit­field modelling methods

We studied theoretically the transition processes that occur during tests of power transformers in the mode of experimental idling. A circuit-field model of electromagnetic processes is developed, based on a three-dimensional dynamical model of the magnetic field in a three-phase power transformer at idling. Using a finite element method, we divided the region of field simulation into estimated zones with the magnetic field symmetry conditions for vertical and horizontal cross sections. The number of finite elements and the time for computing was reduced by four times without compromising the accuracy of our results. It was established that in the circuit of the windings, connected into a triangle, there occurs a transitional levelling current, which fades over the initial switching time. The multiplicity of this current may reach 60‒70 % of the multiplicity of the input surge current. We have developed relations for the approximation of transitional phase currents, which are represented by the aperiodic, periodic components and series with the basis Gaussian functions for phase current discrepancies between the circuit-field and circuit models. We have proposed and implemented a refined approach for the calculation of input surge current based on the specifications for an idling mode of the transformer and a surge current coefficient, which is characterized by the high efficiency and accuracy of numerical realization. To determine a functional dependence of the multiplicity coefficient for an input surge current on the input resistances, we applied a method of sorting out specialized functions. The approximation coefficients calculation was carried out based on the method of least squares. This allowed us to significantly reduce the level of error when calculating the multiplicity coefficient of input surge current based on the specifications of the transformer and testing equipment, by 2.1 %. Using the 3D modeling reduces the calculation error of idling mode current surges by 2.4 % using a simplified procedure that employs specifications of the transformer.

Single-Phase Modeling Approach in Dynamic Harmonic Domain

In this paper, a frequency-based analytical approach is presented for dynamic analysis of three-phase balanced systems in the presence of harmonic distortion based on single-phase analysis. By providing mathematical foundation, this study proves that a three-phase balanced system (linear or non-linear, supplied by periodic balanced sinusoidal or non-sinusoidal sources) is completely balanced during both transient and steady-state conditions. This is done by utilizing Dynamic Harmonic Domain (DHD) and defining a phase-shift matrix in frequency domain. As the most noteworthy application of the proposed methodology, single-phase modeling approach is put forward. Therefore, during the transient period, one can analyze only one phase of a three-phase balanced system and calculate exact quantities of the other phases without performing extra simulations, which is not possible through time domain. The introduced concept has been applied to different test cases including three-phase transformer inrush current. In addition, the proposed approach has been utilized to obtain a single-phase model of VSC-based power electronic devices for dynamic harmonic analysis, followed by discussion on results.

Experimental data-based transient-stationary current model for inter-turn fault diagnostics in a transformer

Inrush current has been well-studied in transformers using different methods to diagnose internal faults. In this paper, a new methodology is proposed using experimental data for obtaining a time-model based on inrush currents. The main contribution of this paper consists of providing a model that accurately reproduces the inrush and steady state currents of single-phase transformers, offering the unique ability to analyze several severity levels of turn to turn faults. The obtained model considers eleven cases of inter-turn fault (2, 3, 4, 5, 10, 15, 20, 25, 30, 35, and 40 short-circuited turns) and allows the study of indices to detect inter-turn faults. The proposed model is based on two functions: a steady-state function based on Fourier series and a residual transient function based on a sum of Gaussian functions. The model allows the study of harmonic content because of its direct time-frequency representation. The model is developed using experimental signals measured from a single-phase transformer, which is validated by means of error criteria that assesses experimental and modeled signals. Its capability to reproduce non-modeled signals with different inter-turn fault cases is also evaluated. The modeling of these signals can be a useful tool for simulation-based applications where a reliable waveform reproduction is needed. On the other hand, indices such as total harmonic distortion, energy spectral density and second harmonic ratio are obtained from the proposed model to study its behavior, depending on the severity of the inter-turn faults. Results demonstrate its usefulness to characterize the level of fault severity.

Demagnetization Method for Reducing Inrush Current of Single Phase 1 kVA Transformer

This study deals with an effort to reduce inrush current of single phase transformer during its energizing. Inrush current affected by residual flux was minimized using demagnetization method in order to reduce residual flux trapped in the core of transformer. Two methods of demagnetization namely variable frequency-constant voltage (VFCV) and variablevoltage–constant frequency (VVCF) were applied and the effectiveness of both methods were then compared. Both methods were supplied with alternated direct current (DC), however the one was changed its frequency and the other one was changed its voltage. The results obtained after transformer was demagnetized using these methods show that VFCV method educed inrush current up to 76%, on the other hand VVCF ethod reduced inrush current only up to 37%. Moreover,demagnetization time process of VFCV method was about 1.46 s, which is 1 s faster than VVCF method.

Inrush Blocking Scheme in Transformer Differential Protection

Transformer inrush currents during energization result in mal operation of transformer differential protection, due to the flow of magnetizing current only on the primary / source side of the transformer. A straightforward approach in recognition of inrush currents is proposed here by checking the core cause of this inrush phenomenon i.e. the magnetic saturation of the transformer core. In this method, the pseudocharacteristics were approximated in terms of orthogonal polynomials, and is used to distinguish between inrush currents and fault currents. In this method, the magnetic saturation characteristics of the transformer is modeled by defining pseudocharacteristics, and is obtained over a half cycle data window of the differential current. This data is assessed using certain criteria to detect inrush and fault currents. This method is not reliant on the transformer models and is precise in its output. The propositioned method was tested using delta-wye transformers under both fault and inrush scenarios.

Automatic J–A Model Parameter Tuning Algorithm for High Accuracy Inrush Current Simulation

Inrush current simulation plays an important role in many tasks of the power system, such as power transformer protection. However, the accuracy of the inrush current simulation can hardly be ensured. In this paper, a Jiles–Atherton (J–A) theory based model is proposed to simulate the inrush current of power transformers. The characteristics of the inrush current curve are analyzed and results show that the entire inrush current curve can be well featured by the crest value of the first two cycles. With comprehensive consideration of both of the features of the inrush current curve and the J–A parameters, an automatic J–A parameter estimation algorithm is proposed. The proposed algorithm can obtain more reasonable J–A parameters, which improve the accuracy of simulation. Experimental results have verified the efficiency of the proposed algorithm.

Simulation of Startup Sequence of an Offshore Wind Farm With MMC-HVDC Grid Connection

This paper proposes a startup sequence for a modular multilevel converter (MMC) HVDC-connected offshore wind farm (OWF) and demonstrates how it can be simulated using an electromagnetic transient-type tool. The objective is to analyze inrush currents of transformers and cables (HVAC and HVDC). Modeling accuracies and computational performances are examined using three existing modeling methods for the MMCs.

Transient current limiter for suppressing transformer inrush, motor starting and fault currents in power system

Transient currents (TCs) including transformer inrush, motor starting and fault currents cause mechanical, thermal and electrical stresses to the transformer windings and the power utility equipments. Of these, the inrush current suppression is crucial due to high second harmonic level, causing inadvertent operation of the protective relays. In this study, a TC limiter (TCL) comprising of two diodes and two reactors in each phase is proposed. The number of diodes in the proposed TCL is the half of those employed in the single-phase diode-bridge type inrush current limiter (ICL); thus, the proposed TCL topology offers much more reliability and lower cost in addition to lower current ripple. Comparing with the three-phase diode-bridge type ICL, the proposed topology is independent of the transformer primary winding connection type. The analysis and design procedure is provided and the performance is verified through simulation and an experimental scaled down prototype for transformer energising, motor starting and fault event.

Identification of inrush currents in power transformers based on higher-order statistics

This paper presents a novel method based on higher-order statistics for discrimination between magnetizing inrush and internal fault in power transformers. The distribution characteristics of the sample points of differential currents, under various operation conditions, are firstly investigated and quantified using higher-order statistics. Based on these characteristics, three kurtosis-based indices are defined for distinguishing inrush current from internal fault current. The final discrimination criterion combines these three indices to improve the performance of inrush current identification. Extensive simulation studies and experimental tests verify the effectiveness of the proposed method and its advantages over the conventional second harmonic restraint method.

Approximate method to compute the maximum rms current in a distribution feeder due to simultaneous inrush currents in downstream transformers

This article shows an approximate method to compute the maximum rms current in a distribution feeder due to simultaneous inrush currents in downstream transformers. In the first part of the article, a method for the exact calculation of inrush currents in distribution transformers (by solving the differential equations) is tested. This exact model was verified with experimental measurements. Subsequently, the exact model was applied in order to develop the proposed method. The approximate method requires two approximations, which only use simple algebraic functions. The first approximation is required to build an equivalent system for the simultaneous inrush currents of transformers in a distribution feeder. This equivalent system is composed by a single transformer, which is fed by an ideal voltage source connected in series with an impedance. The second approximation is required to compute the maximum inrush current of a transformer by assuming that the source impedance is not negligible. These novel approximations were tested for diverse integration time intervals. The results show that the accuracy of the developed method can be considered suitable because the average percentage error, in comparison with the exact model, is in the order of 4% for the first approximation and 5% for the second approximation. The developed method allows a straightforward calculation of the maximum rms current in a feeder due to inrush currents in downstream transformers, and the knowledge of this magnitude is useful for the proper coordination of overcurrent devices in electric power distribution systems.

Controlled switching strategies for transformer inrush current reduction: A comparative study

Transformer inrush current can be efficiently mitigated by controlled switching. For energization of three-phase transformers, several strategies have been developed that depend on whether the residual flux can be measured and whether independent-pole-operated or three-pole-operated circuit-breakers are used. In this paper, a comparative analysis of these strategies has been carried out. Residual flux uncertainty and dispersion of the characteristics of the circuit-breaker can significantly affect the performance of the controlled switching. These issues have been taken into account in the analysis of each strategy since they can influence the performance in different ways. The study has resulted in the selection of the most appropriate strategy for each case.